Character Tendency

Character Tendency in Demon's Souls and Demon's Souls Remake is a special mechanic similar to most common morality systems. Demon's Souls Character Tendency determines the propensity of the player to do good deeds (white tendency) or bad deeds (black tendency). This results in special events becoming available to the player as the game progresses.

Character Tendency is specific for your character. Demon's Souls also features World Tendency, which is affected by the world. Each tendency acts on its own and doesn't rely on the other one.

Playing the game normally without facing many invaders will allow you to reach Pure White Character Tendency by endgame if you have killed all the NPC phantoms. Make sure you get the Ally's Ring from the Monumental at the nexus when you reach it.
The best time to become Pure Black if you don't like that playstyle is when you are near endgame and ready to go to New Game Plus. Get your Black Character Tendency, do the Mephistopheles Quest, and then move to the next cycle
Character and World Tendency won't reset in New Game Plus

Demon's Souls Character Tendency

What is Character Tendency in Demon's Souls?

The world of Demon's Souls has an absolute morality system that gives players a specific alignment: toward Black Tendency (Evil) or white tendency (Good). Not to be confused with World Tendency, the Character Tendency mechanic is specific to the player and the actions of the character throughout your playthrough. If you help other players and vanquish evil from the world, you will become purer and your tendency will become White. If you kill NPCs and invade other players as an evil phantom, you will become darker and your tendency will become Black.

What are the effects of Character Tendency in Demon's Souls?

Character Tendency has unique ramifications that allow the player to meet specific NPCs and obtain special quests. Your Character Tendency Also affects the weapons Demonbrandt, Soulbrandt and Northern Regalia.

Listed below are the various events or changes that can trigger for the player's character in both White and Black World Tendency

Character Tendency		CT Events/Changes
White CT	> 90%	The Monumental gives the Friend's Ring. Phantom Player's attack power is increased. Attack Power of the player in Soul Form is increased.
White CT	>50%	Phantom Player's attack power is increased. Attack Power of the player in Soul Form is increased.
Black CT	< -50%	The player's max HP in Soul Form is decreased. Black Phantom Player's attack power is increased.
Black CT	< -90%	The player's max HP in Soul Form is decreased. Black Phantom Player's attack power is increased. Mephistopheles appears if Yurt, the Silent Chief was freed.

Changing Character Tendency

Towards White

Defeat Black phantom that invaded (Kill a Player Black Phantom in your world or other world).

Defeat Unique Named NPC Black Phantoms which appear only with Pure Black World Tendency. This applies only to these five unique "named" NPC Black Phantoms (Miralda, Scirvir, Rydyell, Satsuki, and Selen). There are other Black Phantoms which won't adjust your character tendency (tested after Patch 1.04) such as the two BPs in the Crystal Gecko nest tunnel in Armor Spider Archstone or the three BPs that appear just beyond the Penetrator's hall Penetrator Archstone. Killing Black Phantoms Ostrava, of Boletaria and Garl Vinland will also not adjust your character tendency, even though they are named.

Gates of Boletaria - Defeat Black Phantom - Executioner Miralda (Note: you need to deliver the final killing blow. Don't let her fall to her death in the stairwell)
The Tunnel City - Defeat Black Phantom - Scirvir, the Wanderer
Prison of Hope - Defeat Black Phantom - Lord Rydell
The Ivory Tower - Defeat Old Monk in Online Mode (counts as Black Phantom PK)
Island's Edge - Defeat Black Phantom - Satsuki
Swamp of Sorrows - Defeat Black Phantom - Selen Vinland (Note: don't kill her with Soulsucker or there will be no effect in Character Tendency - verified by meng-huo from GameFAQs forum)

Note: Killing these 5 Black Phantom NPC in certain manner (details uncertain) will not trigger the shift in Character Tendency. Here are some tested examples:

Killing Black Phantom Selen Vinland with Soulsucker.
Let Black Phantom Executioner Miralda fall to her death in the stairwell. (A blast from Rage of God or fighting in the stair well may cause this)
Let Black Phantom Scirvir, the Wanderer fall to his death
Killing any Black Phantom exclusively with Poison Cloud seems to prevent the shift in character tendency. (I tested this with Rydell and Scirvir, with update 1.04 installed. -StefanieTN)

Towards Black

Kill harmless NPC.
Invade as a Black Phantom, kill host.
Note that if the host dies to anything other than your damage (environmental damage, enemies, fall damage, self-damage from Magic Sword Makoto, Hiltless, or Cursed Weapon) it will not lower your CT.

Here's a list of relatively harmless (won't hurt your game if dead) NPCs that you can kill to shift the character tendency.
(Note: please add to it from time to time.)

Gates of Boletaria, The Lord's Path, Inner Ward - Dregling Merchant
Smithing Grounds, The Tunnel City - Filthy Man
Prison of Hope - Once Royal Mistress
Island's Edge, The Ritual Path - Graverobber Blige
Depraved Chasm, Swamp of Sorrows - Filthy Woman
The Nexus, Gates of Boletaria, The Lord's Path, Inner Ward - Ostrava, of Boletaria (body form)
The Nexus, The Lord's Path, Inner Ward, The King's Tower - Biorr, of the Twin Fangs
Nexus: Blue Phantom NPC (If you don't speak to him, he'll remain in the Nexus for the duration of the game, or until you kill him.)
Killing four or five (depending on World Tendency) will take you from Pure White to Pure Black.

Notes:
Killing the following Named Black Phantom NPCs has no impact (either way) on Character Tendency (both confirmed).

The King's Tower - Black Phantom Ostrava, of Boletaria (He appears during any world tendency.)
Rotting Haven - Black Phantom Garl Vinland (He appears only during Pure White world tendency.)

CT Character Parameters

Character Tendency in Demon's Souls also influences your stats as a Black Phantom and in Soul Form. Listed in the table below, you'll find the following parameters that will influence the various stats for both Black and White World Tendency.

Character Type	WT Category	Max. HP Multiplier	Physical Attack Multiplier	Magical Attack Multiplier	Fire Attack Multiplier	Souls Gained From Multiplier
Black Phantom Player	Black < -50%	1.00	1.10	1.10	1.10	1.00
Black Phantom Player	Black < -90%	1.00	1.20	1.20	1.20	1.00
Phantom Player	White > 90%	1.00	1.20	1.20	1.20	1.00
Phantom Player	White > 50%	1.00	1.10	1.10	1.10	1.00
Soul Form	White > 75%	1.0	1.20	1.20	1.20	1.00
	White > 50%	1.00	1.10	1.10	1.10	1.00
	Black < -50%	0.95	1.00	1.00	1.00	1.00
	Black < -90%	0.90	1.00	1.00	1.00	1.00